Various control systems have been developed for controlling ultrasonic motors. Examples of these control systems include digital controllers for piezo stages, ceramic servo motors, and motor controllers. Some of these controllers do not allow for reference inputs being applied from an external input. In other controllers, the inputs cannot be updated until the motor reaches a fixed reference point, and as a result these controllers do not allow for a time varying input.
U.S. Pat. No. 5,258,694 to Ohnishi et al. discloses a control device for an ultrasonic motor for controlling to drive the ultrasonic motor according to a logical and operation between a control pulse having a duty ratio changing from 100% to 0% according to a moving position of the ultrasonic motor and a driving signal having a resonance frequency of the ultrasonic motor. Accordingly, after starting the ultrasonic motor at a maximum speed, a moving speed of the ultrasonic motor can be gradually reduced until it reaches a target position, and when reaching the target position, the ultrasonic motor can be stopped precisely at the target position.
U.S. Pat. No. 6,894,422 to Kato discloses an ultrasonic-motor control system that includes an ultrasonic motor, a controller which starts the ultrasonic motor by changing a drive frequency of the ultrasonic motor from an initial drive frequency, a calculation device which calculates an initial drive frequency data based on a drive frequency at the commencement of rotation of the ultrasonic motor, and a setting device which sets the initial drive frequency based on the initial drive frequency data at a subsequent commencement of driving of the ultrasonic motor.
U.S. Pat. No. 6,984,919 to Iino et al. discloses a control system provided with a positioning mechanism including plural ultrasonic motors in the device, which can keep good operational stability as compared with a conventional system with a servo mechanism, is of an energy saving type suppressing power consumption and is compact in structure including the motor and a control circuit therefor. A control circuit in a control system uses a common drive control circuit for a plurality of ultrasonic motors among the positioning mechanisms in which when the drive control circuit inputs a signal that specifies any ultrasonic motor and a desired-position designation signal, the drive control circuit conducts control operation that transmits an on signal to a driver of the subject motor to start the driving, and stops the driving when the motor reaches a desired position while receiving a position detection signal. Thus, the control system has a function of transmitting a signal indicating that the positioning operation is being conducted from a time point when commands are received to a time point when the positioning operation is completed. Then, an error signal is outputted when the motor is out of the desired position.
U.S. Pat. No. 8,253,370 to Araki discloses a microcomputer that controls an ultrasonic motor and includes a storage unit that stores a compare register value, and a digital/analog (D/A) conversion set value, a D/A converter that generates an amplitude control signal with an amplitude value corresponding to the D/A conversion set value, a timer that generates a pulse width modulation (PWM) signal with a frequency corresponding to the compare register value, a central processing unit (CPU) that reads the D/A conversion set value, and the compare register value from the storage unit, and that sets the D/A conversion set value and the compare register value to the D/A converter and the timer, respectively, and an output circuit that generates the control signal with the amplitude of the amplitude control signal, and the frequency of the PWM signal, in response to the amplitude control signal and the PWM signal.
It is an object to provide a novel control system for ultrasonic motors.